Add docs for non-html route encoding option

[altano]

What kind of changes does this PR include?

This is to document a PR I'm publishing to astro core that adds the encoding option to non-html routes in SSG mode.

Description

This PR adds documentation to complement withastro/astro#4549. The doc change looks like this:

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[altano]

Link to new doc content in preview: https://deploy-preview-1434--astro-docs-2.netlify.app/en/core-concepts/astro-pages/#non-html-pages

[sarah11918]

Hi @altano, thanks for submitting some documentation to go along with your feature PR withastro/astro#4549 !

I've converted this PR to a draft so that it does not get merged before the corresponding feature does, as we do with all docs that are awaiting approval elsewhere first. When that PR is merged, this can be un-marked as draft and reviewed at that time. Just so you know what's happening here! 😄

[sarah11918]

Hi @altano! I can see that the upstream PR in the Astro repo looks like it will be merged for an upcoming release, so we can start editing it here! 🥳

I'm leaving it as a draft so that it does not get merged before the release, and you might notice the merge in the other repository before I do, so feel free to come back and comment here to let us know when it's safe to un-draft this one. 😄